A cascade of extreme weather events fed Northern California’s wildfires that exploded Sunday: Unusually high winds blew flames through unusually dense and dry vegetation, which sprung up following last winter’s heavy rains and then were toasted by months of record hot temperatures.

“The dice was really loaded because of the big wet winter,” said Park Williams, a California native and a research scientist at the Lamont-Doherty Earth Observatory at Columbia University. “That set up the West with a lot of fuel to burn, and this summer has been exceptional in terms of dryness.”

Scientists such as Williams say California is especially prone to wildfires, in part because of the state’s dense population, which makes it easy for sparks to be ignited and turn into raging fire storms. But this week’s blazes also show the fingerprints of climate change, he said, a harbinger of what the West should expect in the years to come.

“The fingerprint is definitely there,” said Williams, who last year contributed to a study on climate change’s impact on western wildfires. “The connection between temperatures and fire is one we see again and again in the correlation analyses we do.”

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California’s fire chief said he and other firefighters were stunned by the fury and speed of the blazes that erupted Sunday night. “We’ve raised the bar again in California just in terms of the conditions that we’re facing and the destruction and devastation, ” Cal Fire Chief Ken Pimlott said Monday.

As of Tuesday morning, at least 15 people had died and more than 1,500 homes were destroyed in the multiple fires, including one that destroyed much of northeast Santa Rosa, a city of 175,000 people. There, the Tubbs fire incinerated hotels, a high school, a mobile home park and vast neighborhoods, the worst one-day wildfire destruction in California since the Oakland Hills fire of 1991.

The fingerprint is definitely there

Park Williams, research scientist at at Columbia University, referring to the impact of climate change on this year’s wildfires.

Meteorologists term the winds that struck Northern California Sunday as “Diablo winds,” similar to the Santa Ana’s that contribute to large wildfires in southern California nearly every fall.

In both situations, inland high pressure combined with offshore low pressure draws dry winds in from the northeast. As these winds race down the slopes of the Sierra Nevada, they compress and become warmer and stronger.

On Sunday night, winds reached 50 miles per hour near Santa Rosa, with some gusts topping 70 miles per hour, according to the National Weather Service. The surface dew points, a reading of moisture in the air, were in the mid teens, lower than values in Las Vegas and Phoenix, according to Jon Erdman, a senior meteorologist with Weather.com.

It is unclear how the multiple blazes started, but even small sparks from a car or campfire could have ignited the fires, given the winds, said Williams.

Once ignited, the fires jumped from one patch of vegetation to another, much of it bone dry because of this summer’s record temperatures. On Sept. 1, Santa Rosa recorded its hottest day of the year, 110 degrees. That surpassed the city’s previous Sept. 1 record of 105 degrees, set in 1950.

The combination of a wet winter followed by extreme heat and dryness has caused record wildfires in many Western states, including Montana, where more than 1 million acres burned. The federal government estimates it spent more than $2 billion fighting wildfires nationwide this year.

In terms of U.S. acreage burned, nine of the 10 worst fire seasons in the past 50 years have occurred since 2000, according to the National Interagency Fire Center. Foresters attribute part of that increase to past fire suppression policies, which have resulted in forest unnaturally thick with underbrush. But rising temperatures are also a factor.

Last year, Williams contributed to a study that found that human-induced climate change had doubled the area affected by western forest fires the last 30 years. The study, which appeared in the Proceedings of the National Academy of Sciences, concluded that higher temperatures and increased aridity caused an extra 16,000 square miles to burn beyond what would otherwise would have caught fire.

The study, led by John Abatzoglou of the University of Idaho, warned that western wildfires could increase exponentially in size with future expected warming.

Williams, who grew up in Folsom, Ca., cautioned that the 2016 study focused on wildfires in western forests, as opposed to those that erupt in grasslands and the “urban wildland interface,” such as the outskirts of Santa Rosa.

He also said that California is particularly unique in that its large populations allows wildfires to ignite more frequently than they would otherwise. “Humans introduce a lot of chaos into otherwise natural fire events,” he said.

Yet even in California, said Williams, climate change is compounding the risks of wildfires by extending the length of the fire season and adding to the intensity of droughts and heat waves. “I’m highly confident that the combination of dry fuel, extreme heat and climate change is a recipe for what we are seeing,” he said.